1. Field of the Invention
The present invention is related to the field of wireless communications between devices that participate in forming a wireless network, and more specifically to devices, softwares and methods for advancing scheduling of a next contention session in a wireless network if a scheduled exchange terminates before its scheduled window.
2. Description of the Related Art
Local area networks (LANs) are increasingly used to transfer data. A relatively new application is wireless LANs, also known as WLANs. These can provide the benefits of a wired LAN, without requiring the different stations to be physically coupled to each other. There is no need for procuring transmission wires such as coaxial conductors, twisted pairs of wires, optical fibers, etc. for transferring the data. Instead, the data is transferred through space, either using radio frequency (RF) waves (that are also known as microwaves), or optical frequency waves, such as infrared (IR) light. A network may be formed by bringing components close together, without the need to plug transmission wires to them.
Referring to FIG. 1, an example of a WLAN is shown. A Hybrid Coordinator device HC 120 is a device that performs admission control and schedules transmission opportunities. Device HC 120 is brought close to a first wireless station STA1 140, a second wireless station STA2 150, and a third wireless station STA3 160. HC 120 may establish a wireless communication link 145 with STA1 140, a link 155 with STA2 150, and a link 165 with STA3 160.
Wireless links 145, 155, and 165 share the same medium 168, which is typically air. Every device in the medium can receive what the others are transmitting. If many are transmitting at the same time, there would be problems (corrupting of data, etc.). Accordingly, only one should be permitted to transmit at a time.
Concurrent transmission is typically avoided by requiring all devices HC 120, STA1 140, STA2 150, STA3 160 to comply with protocols. Many such protocols are being developed under the aegis of the Institute of Electrical and Electronic Engineers (IEEE). An applicable protocol for WLANs is being developed in terms of IEEE Standard 802.11.
Referring to FIG. 2A, operation of at least one protocol is described. A time axis TM designates times of the events. Pulses shown above the time axis TM are those transmitted by HC 120, while those below axis TM are transmitted by devices STA1 140, STA2 150, STA3 160. Devices STA1 140, STA2 150, STA3 160 are collectively designated as STAX.
The protocol works by having HC control when each device may transmit. As time goes on, HC 120 periodically establishes intervals MC of contention for the medium 168, followed by polling pulse intervals P, followed by contention free intervals CF.
During medium contention intervals MC, devices STA1 140, STA2 150, STA3 160 are free to transmit requests for use of the medium 168, at the expense of each other. These are requests for reserving resources, such as bandwidth and memory, and are therefore also known as reservation requests.
Afterwards HC 120 processes the contending reservation requests, and resolves them. In other words, it generates a schedule for when each one of devices STA1 140, STA2 150, STA3 160 should transmit.
During the polling pulse interval P, HC 120 announces the schedules of which device (or pair of devices) may transmit during the next contention free interval CF. Announcing is by usually transmitting a poll frame, which is referred to as “QoS CF poll frame” in the IEEE 802.11e standard. All devices STA1 140, STA2 150, STA3 160 receive it, and all must comply.
During a contention free interval CF, only those devices scheduled by HC 120 may transmit data. The others must neither exchange data, nor contend for the medium 168. A single contention free interval CF may be scheduled for a session of only one exchange at a time. Sometimes there can be two or more exchanges scheduled in a row in a single contention free session. Each exchange may be between two devices that exchange data, acknowledgement pulses, etc. But before contending again, devices STAl 140, STA2 150, STA3 160 wait for the next medium contention interval MC.
A contention free interval is scheduled for time TCF. Afterwards there is a minimum time interval TJ that must be observed, before contention begins again. Time interval TJ must be of a very short time duration. This way the continuity will not be broken, and any other device that may want to contend for medium 168 will have to continue waiting.
Radio based WLANs have several characteristics which differ from those of wired LANs. These characteristics include lower achievable data capacity, which is due to a number of factors, such as bandwidth limitations. That is why it is desirable to not waste time in such communications.
Referring now to FIG. 2B, a situation is shown where time is wasted. Everything is the same as in FIG. 2A, except that the transmissions during the first contention free window “CF” last only for time period T11, which is less than the allotted TCF.
This may happen for a number of reasons. For example, the amount of the data in the buffer may have needed only a shorter transmission opportunity. Or one of the devices STA1 140, STA2 150, STA3 160 may have used an acknowledgement policy, but has not received any, and thus ceased transmission. Or simply the device has lost its association.
When that happens, this leaves a quiet time window TQ. It is quiet because no device is transmitting, as every device is waiting for another device to transmit.
Quiet time window TQ is merely wasted time at best. Worse, window TQ may be larger than the duration of TJ, or a designated Short Inter Frame Space (SIFS) of the connection. This engenders the possibility that a device may seek to establish a connection with HC 120. Such a device may either be a non-compliant device, or even a compliant device that has been brought close, and may fairly interpret the long duration of quiet time window TQ as a suitable opportunity to submit a reservation request.
This way the new device will start contending at the expense of the established ones. It may win, because the existing devices which will not even be contending. This will break the continuity, and force rescheduling, which is inefficient. In the worst case, there could be collision.